The present invention relates generally to D.C. electric motors, and particularly to the speed control of separately excited or shunt wound D.C. electric motors.
The speed of a D.C. electric motor may be varied by changing the field strength of the motor, by changing the armature voltage, or by inserting a resistance in the armature circuit. For example, the speed of a separately excited or shunt wound motor may be increased by weakening the field, which lowers the counter E.M.F. (electromotive force) generated by the armature to maintain a balance of the load, counter E.M.F. and applied E.M.F. Although a variation in the field strength provides a constant horsepower, the torque of the engine will vary over the speed range when the armature current is held constant. Accordingly, the motor will produce a high torque at low speeds and a low torque at high speeds. This characteristic of a shunt wound or separately excited motor causes a problem when the load on the motor is suddenly increased, because the speed of the motor will decrease if the torque is insufficient to meet the new demand. This is particularly disadvantageous when the motor is used as a propulsion motor for an electric vehicle, where a high torque would be required for climbing a hill or passing another vehicle.
Another aspect of controlling the speed of a motor by varying the strength of the field is the speed range available. A typical shunt wound or separately excited motor will provide a speed up to 2.5 times the rated speed by weakening the field. However, in order to get a substantial decrease from the rated speed, it is necessary to lower the voltage in the armature circuit, because the strength of the field cannot be increased beyond its designed characteristics. Providing for an adjustment in the armature voltage is considered undesirable, as this will increase the complexity and cost associated with the motor controller. In the case of a shunt wound motor, the speed range is further constrained, because the applied E.M.F. is the same for both the armature and the field. Accordingly, when the applied E.M.F. is adjusted to vary the strength of the field, the current through the armature varies as well.
The present invention provides a novel and improved shunt wound or separately excitable D.C. electric motor which has a wide speed range available and is capable of producing a high torque at high speeds. Particularly, the motor features a novel stator assembly, which generally includes a pair of field poles, each having a field coil wound around a shank portion, and a plurality of separately excitable field control coils wound individually around the field pole legs defining a face portion of the field poles. An important aspect of the present invention is a core portion of the field pole, which provides a magnetic path between the shank and face portion. This core portion permits the magnetic flux in the field poles to shift across the face portion when one or more of the field control coils are energized magnetically subtractive to the flux generated by the field coils, as will be more fully described below.
The present invention also provides a novel method of controllng the speed of the shunt wound or separately excited motor. In a starting condition, all of the field control coils are energized magnetically additive to the flux generated by the field coils. The starting condition represents the lowest operating speed of the motor, as the field coils and field control coils cooperate to provide the strongest capable field. The speed of the motor may be increased from this starting condition by de-energizing the field control coils in a predetermined sequence. As each field control coil is de-energized the field is weakened, thereby lowering the counter E.M.F. and causing the speed of the motor to increase in gradual steps. The speed of the motor may be further increased by energizing the field control coils in a predetermined sequence, magnetically subtractive to the flux generated by the field coils. This causes the magnetic flux in the field poles to shift across the face portion, such that the flux generated by the field coils will shift away from the subtractively energized field control coils. Thus, if the field control coils are subtractively energized from top to bottom, the field coil flux will shift from top to bottom away from the energized field control coils. This effectuates a reduction of the number of active armature coil turns, thereby lowering the counter E.M.F. generated by the armature. As the counter E.M.F. decreases, the speed of the motor will increase in order to maintain a balance of the load, counter E.M.F. and applied E.M.F. Accordingly, as each of the field control coils are subtractively energized, further gradual increases in the speed of the motor may be achieved. The speed of the motor may subsequently be decreased by reversing the sequence of steps just described.
It will be appreciated by those skilled in the art that a wide speed range may be achieved with the novel motor and speed control method according to the present invention. The number of speed gradations will, of course, be dependent upon the number of field control coils provided in each of the field poles. It should also be appreciated that in addition to controlling the polarity of the field control coils, the voltage applied to these coils may also be varied to provide further and more continuous gradations in speed.
As stated above, another advantage of the present invention is the capability of providing a high torque at high speeds. When the field control coils are subtractively energized in a predetermined sequence, the magnetic flux in the field poles will shift across the face portion, thereby concentrating the flux lines over a narrow portion of the pole faces. This narrow, but strong flux pattern provides a high torque across the active turns of the armature.
The present invention further provides a novel motor controller, which is particularly advantageous when the motor is used to drive an electric vehicle. In addition to providing speed control circuitry for a motor in accordance with the present invention, the motor controller also provides regenerative control circuitry for re-charging the electric vehicle battery during de-acceleration. When the vehicle accelerator is released, the motor controller will energize all of the field control coils magnetically additive to the flux generated by the field coils. This will cause the counter E.M.F. to rise above the applied E.M.F., resulting in a charge being returned to the battery as the vehicle slows down. Thus, during de-acceleration the motor acts as a generator, re-charging the battery and providing a dynamic braking force for the vehicle.
Other features and advantages of the invention will become apparent in view of the drawings and the following detailed description of the preferred embodiment.